A battery is largely classified into a primary battery and a secondary battery. The primary battery produces electricity via an irreversible reaction and thus cannot be reused after being used once. An example of the primary battery includes a commonly used battery, a mercury cell, a voltaic cell, and so on. On the other hand, the secondary battery uses a reversible reaction and thus can be recharged and reused after being used. An example of the secondary battery includes a lead storage battery, a lithium ion battery, a nickel-cadmium (Ni—Cd) battery, and so on. FIG. 1 is a diagram conceptually illustrating a structure of a typical lithium ion battery as a secondary battery. A lithium ion battery and a lithium ion polymer have the same structure except for appearance (liquid/solid) of electrolyte. In addition, materials of electrodes or electrolyte may slightly differ from FIG. 1 according to a battery. As illustrated in FIG. 1, the lithium ion battery is composed of an anode 1 generally formed of carbon, a cathode 2 generally formed of a lithium compound, an electrolyte 3 disposed between the anode 1 and the cathode 2, and electric wire 4 for connection between the anode 1 and the cathode 2. During charging, lithium ion in the electrolyte 3 moves toward the anode 1, and during discharging, moves toward the cathode 2. In this case, each electrode causes a chemical reaction while discharging or absorbing surplus electrons. During this process, electrons flows in the electric wire 4 to produce electric energy. Although the lithium ion battery has been described, other secondary batteries have the same basic principle and structure while only materials of electrodes or electrolytes are changed from the lithium ion battery. That is, in general, as described above, the secondary battery includes the anode 1, the cathode 2, the electrolyte 3, and the electric wire 4.
In this case, the secondary battery may include a single anode 1, a single cathode 2, a single electrolyte 3, and a single electric wire 4. Alternatively, in general, the secondary battery may includes a plurality of unit cells each composed of a single anode 1, a single cathode 2, a single electrolyte 3, and a single electric wire 4. That is, the plural unit cells 10 are contained in a secondary battery pack. Needless to say, the unit cells 10 are electrically connected to each other.
In general, the secondary battery includes a plurality of unit cells therein and is configured to externally expose one pair of external terminal tabs connected to electrodes of each cell (i.e., which is one pair of taps included per battery and functions as one anode connected to negative cells of each unit cell and one cathode connected to positive cells of each cell). In general, with regard to such a secondary battery, the plural secondary batteries instead of a single secondary battery are connected to each other to constitute a battery as one pack. Each battery of this type of battery pack is referred to as a cell (different from a unit cell included in a battery). Needless to say, tabs of each cell are electrically connected to each other.
In general, a battery is formed by stacking a plurality of cells and electrically connecting tabs of the cells to each other. In this case, methods such as bolt fastening, welding, and so on in order to connect the cells to each other. According to bolt fastening, problems arise in terms of the increased number of components and assembling processes. Thus, cell connection using welding has been widely used, in accordance with current trends. Various methods such as ultrasonic welding, laser welding, resistance welding, and so on may be used for welding for cell tab connection.
In general, a tab of a cell is divided into a cathode tab and an anode tab that are normally formed of different metals. For example, in a secondary lithium battery, in consideration of electrochemical stability, the cathode tab is formed of aluminum (Al) and the anode tab is formed of copper (Cu) or Cu plated with nickel (Ni). Needless to say, in order to increase electrochemical reactivity and stability, in most cases, the cathode tab and the anode tab are formed of different metals.
However, as described above, when cell tabs are welded to each other, since the cathode tab and the anode tab are formed of different materials, problems may arise in that joining cannot be smoothly achieved due to different physical properties. When joining cannot be smoothly achieved during welding, problems arise in terms of reduced durability and increased contact resistance. In addition, uniform welding intensity cannot be achieved which causes large deviation of product performance.
However, so far, researches have not been conducted to overcome these problems for battery manufacture. Accordingly, there are increasing demands and expectations of those of ordinarily skilled in the art for overcoming problems in terms of welding errors caused by different physical properties between positive and anode tabs.